Influence of Cooling Rates on the Dentrite Morphology and Hardness of the Surface-Chilled Mg-10Gd-3Y Magnesium Alloy

Abstract:

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Mg-10wt%Gd-3wt%Y alloy was cast in a step-like mould, which provided five different
cooling rates. The dentrite morphology and hardness of the as-cast alloy from the surface to the center
was investigated and the influence of the cooling rates on these was analyzed. It was indicated that
there were two different trends for the hardness distribution: in the section of the step castings with
the two slowest cooling rates, the hardness decreased with the increase of depth; while in the other
three step castings the hardness increased with the increase of depth. Also it was founded that the
hardness could be influenced by the grain boundary, dentrite morphology and dentrite arm spacing
(DAS) in the alloy. At last, according to the standard deviation curves of the hardness, the chilled
depths were about 5mm in 1st step and 10mm in 2nd step. And the other steps were fully chilled.

Abstract: Ceramic coatings were synthesized on pure Mg and binary Mg-7.1wt%Al alloy by plasma
electrolytic oxidation (PEO) technique, and the effect of current ratio, C2/C1, ranging from 0.2 to
0.85 on their mechanical and electrochemical properties was investigated. As the C2/C1 ratio
increased, the thickness of the coating layer increased, while surface roughness was almost
unchanged. The hardness and wear resistance had a marked tendency to increase with increasing
C2/C1 ratio and Mg-Al alloy showed higher hardness and wear resistance at all C2/C1 ratios
compared to pure Mg. The weight loss of the pure Mg and Mg-Al alloy in 3.5%NaCl solution was
reduced by coatings. However, it had no dependence on the C2/C1 ratio. The wear resistance of the
coated Mg-Al alloy was better than that of the coated pure Mg.

Abstract: ZL114A was joined by friction stir welding and liquid nitrogen was used to cool the welded joint during the welding process. The Influence of forced cooling on the microstructure and properties of welded jointed was studied. The results show that, when forced cooling is used, the amount of silicon particles in weld nugget zone is larger and the size is more homogeneous, the size of silicon particle in heat-mechanical affected zone is more fine , the grain size of heat affected zone is half of air cooling welded joint and the amount of Al-Si eutectic is smaller than air cooling welded joint. The micro hardness increases after forced cooling is used, the hardness of weld nugget zone is 20Hv higher than air cooling welded joint. The strength of forced cooling welded joint increases 13.6% than air cooling welded joint.

Abstract: Strip casting process is a kind of promising near net shape technology for producing magnesium alloys strip. The microstructure and mechanical properties of CO2 laser welded as-cast AZ31 magnesium alloy sheet were investigated. The microstructure of base material (BS) was typical columnar dendritic structure. A significant grain refinement occurred in the fusion zone (FZ) due to the rapid cooling, while an obvious grain coarsening occurred in the heat affected zone (HAZ). The tensile strength and elongation at fracture of weld joints were reduced due to the weld depression caused by high heat input. The hardness in the FZ was the highest due to fine grains, while there was a decrease in hardness of the HAZ, but little variation compared with the hardness of the BS.

Abstract: The microstructures and kinetic characteristics have been studied during heat treatment of AM60B alloy prepared by thixoforming. The results indicate that primary particles and secondary grains coarsened rapidly in initial 1 h solution treated at 430°C, and then slowed down. Their growth modes were different in the initial 1 h and then turned to be same. Accordingly, the alloy hardness dropped quickly before 1 h. Ageing hardness depends on both content of precipitates and the morphology. Adding content of the precipitates hardened the alloy till 8 h aged at 215°C after 24 h solution treatment at 430°C, and then the precipitates coarsening weakened the ally.

Abstract: AZ80 magnesium alloys were deformed at different temperature (270°C, 300°Cand 330°C)with different deformation ratio from 10% to 50%. The influence of varying the deformation temperature and ratio on the microstructure and hardness of AZ80 magnesium alloy was studied. The experimental results show that the hardness increased with the increasing of deformation and the hardness is up to the peak value with 40% deformation at 300°C. The microstructure was homogeneous and the grain was refined after hot deformation.The roles of both deformation strengthening and dispersition strengthening were to im prove the mechanical property of AZ80 magnesium alloy.